A significant cause of wear of metallic extrusion processing component parts is mixing or transporting of material by two co-rotating or counterrotating processing elements. Wear manifests itself in corrosion or abrasion of exterior working surfaces of the component parts. High wear, corrosion resistant extrusion processing component parts often are made from powdered metal (PM) tool steels. A bimetallic material design is used to minimize the amount of PM material needed to make the component parts. In the case of an extrusion screw element, the bimetallic material includes an inside core, with a layer of PM tool steel consolidated around the core by pressure applied by hot isostatic pressing (HIP). The core is typically made from a round bar of mild steel, such as AISI 1018 (low carbon) mild steel or stainless steel.
The following is a brief description of a conventional process of manufacturing an extrusion screw element. The mild steel round bar is placed inside of a cylindrical container (typically made of mild steel) and supported within the container by two end caps. Filling stems are attached to one of the end caps. The container is seal-welded together and checked for leaks to verify that it is hermetically sealed. After the container is assembled, the PM material is delivered through the filling stems to fill the space inside the container between its inner wall and the outside surface of the core material. Once the container is filled with PM material, vacuum is pulled on the container. As vacuum is achieved, the container assembly is heated to facilitate removal of contaminants. The container is sealed when specified temperature and vacuum levels are reached. The sealed container is then placed in a HIP furnace and consolidated into a fully dense material that is composed of the PM material fully bonded to the core material. During the HIP cycle, the container deforms as the PM material densifies so that the final assembly becomes substantially free from voids. This process is called running a HIP cycle, in which materials are bonded in a solid state.
The container is cut away from the HIP processed material. Exterior working features in the form of material conveying or transporting flights of the extrusion screw elements are machined from the outer PM layer of HIP processed material. Machining entails whirling, milling, or turning operations. Before finish machining is performed, the extrusion screw elements are heat treated so that the PM material becomes hard and abrasion resistant and the core material remains soft.
The above-described extrusion screw element manufacturing process wastes a significant amount of PM material. The flights of a typical extruder conveying screw are created by machining a helical channel into the outside surface of the conveying screw. (Conveying screws have one or more screw flights, and most conveying screws have two screw flights.) The helical channel is typically machined along the entire length of the conveying screw. Extrusion screw processing component parts of this type are made for a wide range of extruder sizes (diameters). A larger diameter extruder conveying screw produces a larger amount of wasted PM material during fabrication. When they wear out, the PM extruder conveying screws are discarded because the PM material is not weld repairable.
Plasma transfer arc (PTA) weld overlay is another method of making extruder conveying screws. PTA weld overlay entails creating a hard outer layer of wear, corrosion resistant material at the tips of the flights on the outside surfaces of the extruder conveying screws. This is achieved by first machining a helical groove into a length of weldable round bar. The helical grooves serve as a weld preparation area in which the PTA weld overlaying process is performed. After the welding has been performed, the screw flights are machined into the outside surface of the extruder conveying screw and timed so that the tips of the screw flights land in the overlaid material. Another method entails machining the extruder conveying screws first and then welding the hard outer layer of material to the extruder conveying screws after they have been machined. This method generally requires touchup machining after the flights have been welded. The PTA process creates an extruder conveying screw that has hard material at the tips of the flights, but the remainder of the flights has no protection against wear or corrosion. Screw flights made using the PTA weld overlay process can be restored for reuse, but only with weldable restoration materials. This limitation precludes use in the PTA process of many alloys that exhibit one or both of wear and corrosion resistance properties.